Hanna Instruments HI 903 - Manual (Page 137)

9-9
METHODS OPTIMIZATION
9.2 The Sample
9.2.1 Proper Sampling Procedure
Proper sampling is essential for accurately determining the water content of bulk materials,
particularly with non-homogeneous samples. Many standard methods detail instructions to
ensure proper sampling. As a general rule, the following guidelines should be followed:
1. The sample must be representative. The water content of the sample taken is the
same as the average water content of the bulk material.
2. Avoid exposing samples to the contaminating effects of atmospheric moisture.
Take samples as quickly as possible and protect the sample during transport and/or
storage.
3. Take samples from the interior of bulk materials. Surfaces of hygroscopic materials
may contain higher levels of moisture relative to the rest of the material. Surfaces
of materials which release water may contain less water relative to the rest of the
material.
4. Taking large samples of bulk materials will result in a more representative sample.
9.2.2 Determining the Optimal Sample Size
The proper choice of sample size is critical to achieving accurate and repeatable titration
results. As a general rule, the sample size should be selected such that about 30-70% of the
burette volume is consumed during a titration. This provides enough titrant to ensure good
accuracy while conserving reagents and minimizing the generation of waste.
The table below illustrates the relationship between titration reproducibility, the volume of
titrant consumed during a titration, the amount of water contained in a sample, the size of a
sample and the water content of a sample.